The present invention relates to a radiation sensor, which has particular, but not exclusive, application as a solar radiation sensor for use, for example, in recording duration and/or intensity of sunshine.
For many years, hours of sunshine has been part of the range of meteorological information recorded at monitoring stations throughout the world. The most widely used meteorological recorder is the Campbell-Stokes recorder which uses a spherical lens to focus the sun""s rays to a point to burn a track on a strip of recording paper. Use of a Campbell-Stokes recorder is labour-intensive because the recording paper must be replaced manually each day.
Weather recording has on the whole become more automated, with most measurements being made by automatic instruments which can, if need be, operate entirely unattended and send their data to a remote monitoring station. As a result of this, there is a disincentive to equip a weather station with a Campbell-Stokes recorder since doing so would negate many of the benefits achieved through automation of other measuring instruments. This has meant that there is a growing gap in the climate record for sunshine information as existing Campbell-Stokes recorders are discarded and not replaced. This is occurring at a time where accurate climatic knowledge is vital to enable monitoring of climate change, and during a period of growing interest in solar power, and optimisation of crop growth in agricultural sciences, all of which applications benefit greatly from sunshine data.
There have been attempts to provide an alternative type of sunshine sensor. These sensors have, for the most part, operated by comparing the outputs of two detectors, one of which is shaded from direct sunshine and the other of which is in open sun. Shading of one of the detectors is usually done with a shade ring, which is set to obscure the track of the sun across the sky as seen by the shaded detector. Correct positioning of the shade ring varies seasonally as the solar declination varies by over 46xc2x0 from June 21st to December 21st, and is dependant upon many variables including the latitude and longitude of the sensor""s location and proper alignment with true north. Moreover, improper positioning of the shade ring can give results which are incorrect, but which might not appear to be obviously wrong so that errors could go unnoticed. As a result, this type of instrument is not suited to unattended operation.
An aim of the invention is to provide a recording instrument which can make accurate measurements of direct and diffuse sunlight, and calculate sunshine hours from this data, and which can operate with a minimum of manual intervention.
According to a first aspect of the invention there is provided a sunlight sensor comprising at least two light sensitive detectors and a masking element, and characterised by the masking element having a pattern of translucent and opaque areas which are disposed to ensure that at any time at least one detector can be exposed to direct sunlight through a translucent area and at least one detector is shaded from direct sunlight by an opaque area.
In conditions of sunshine, regardless of the position of the sun in the sky, a great amount more light will be received by a detector which is exposed to the sun through a translucent area than will be received by a detector which is shaded from direct sunlight by an opaque area of the masking element. All of the detectors also receive light from all parts of the sky away from the sun. In cloudy conditions, the difference in the amount of light received by two such detectors will be less. This is true irrespective of the absolute intensity of the sunlight.
The opaque areas of the masking element may be provided by a cut out mask of card, metal, plastic sheet or any other suitable material (the translucent areas thus being nothing more than absence of such material).
The masking element may be disposed within an at least partly translucent cover of the-sensor.
In a particularly preferred arrangement, opaque areas of the masking element are constituted by an opaque pattern applied to an at least partly translucent cover of the sensor. In such embodiments, the pattern of opaque areas can be formed, for example, by coating an internal surface of the cover in selected areas by printing, painting or spraying an opaque coating onto the internal surface, or applying thin opaque material thereto. The cover may include a hemispherical dome of, for example, transparent plastic material.
The light sensitive detectors may be arranged in a symmetrical pattern. They may be carried on a generally flat support. In embodiments in which a cover is provided, it preferably covers all of the detectors.
A sensor embodying the invention may have two light sensitive detectors, but more commonly has a greater number, six or seven being typical. In embodiments which have six detectors, they may be arranged equiangularly and equidistantly spaced from a centre point. In an alternative embodiment, six detectors may be disposed in such a manner, with a seventh placed at the centre point. Clearly other symmetrical or asymmetric arrangements may be used.
The pattern of translucent and opaque areas of the masking element may comprise alternating translucent and opaque bands extending as areas of a hemispherical locus. The translucent and opaque areas preferably are of equal area.
The translucent areas may be nothing more than areas in which a light-obscuring medium used to form the opaque areas is absent. The opaque areas may be symmetrical with respect to a diametric line of symmetry. Alternatively, the pattern of opaque areas may be such that areas in one diametrical half of the hemispherical locus are symmetrically opposite translucent areas in its other half. In embodiments which include a dome-shaped cover, the hemispherical locus typically is coincident with an inner surface of the dome.
A sensor embodying the invention does not require directional alignment in order to operate properly, but needs only to be horizontally levelled. The pattern of opaque and translucent areas is such that the orientation of the sensor is irrelevant to the operation of the sensor.
In another of its aspects, the invention provides a method of distinguishing between sunshine and overcast weather conditions characterised by placing a sensor according to the first aspect of the invention in a position where it is liable to be exposed to sunlight, assessing signals derived from each of the detectors, and comparing the signals derived from the detector exposed to the greatest and to the least amount of light.
The operating principle of the present invention can be applied more generally to create a sensor to distinguish between conditions of diffuse and point source of a wide range of different types of radiation. Therefore, from yet another aspect, the invention provides a radiation sensor comprising at least two radiation sensitive detectors and a masking element, and characterised by the masking element having a pattern of areas which are opaque to the radiation and which are transparent to the radiation which areas are disposed to ensure that at any time at least one detector can be exposed to direct radiation through a non-opaque area and at least one detector is shaded from direct radiation by an opaque area irrespective of where a point source of radiation might be positioned within a detection field.
By provision of suitable detectors and material from which the opaque areas may be made, such a sensor can be used ti detect many different types of radiation including radiation in a large part of the electromagnetic spectrum and radiation of subatomic particles.